This invention relates to machine tools, and more particularly to a driving system for an automatic lathe having a work spindle and at least one rotary toolholder, the axis of which is parallel to or the same as that of the spindle, the system being intended to drive a rotary tool for carrying to end-machining operations on the end face of a workpiece.
End-machining of a workpiece, e.g., one formed at the end of a bar, in the headstock of an automatic lathe is currently carried out according to one of the following two methods, both of which present drawbacks:
In the first method, the work spindle is stopped when end-machining is to be carried out, and the tool carriage is moved axially while the tool is rotated. The drawback of this method is the loss of machining time due not only to the time necessary for stopping the workpiece and then restarting it, as the case may be, but also to the fact that it is not possible to carry out any turning operation on the workpiece during the end-machining operation.
In the second method, the end-machining tool and the workpiece are both rotated, each by its own motor, the actual machining speed equalling the sum of or the difference between the speeds of rotation of the workpiece and the tool. With this method, the above-mentioned drawbacks are avoided. However, the driving power necessary for the motor rotating the tool is greater than with the first method, for the tool must be supplied with a certain machining torque which depends on its size, on the metal of which the workpiece is made, and on the desired operation. If this torque must be supplied at a speed close to the speed of rotation of the workpiece, which can reach several thousand rpm, the power necessary for driving the tool is increased as compared with the power necessary when the workpiece is stationary, and the ratio of the powers is the same as that of the speeds. Furthermore, the power of the driving motor must also be increased in order to keep the tool from being driven at the speed of rotation of the workpiece during machining.
In automatic lathes of the profiling type, it has been proposed to establish a mechanical connection between the means for driving the workpiece and the means for driving the tool in order to be able to have both elements driven by one motor. This solution is complicated and expensive and requires a great deal of preliminary work before each operation.
Hydraulic circuits comprising pumps and hydraulic motors have already been used for controlling machine tools. Thus, U.S. Pat. No. 2,036,821 describes a hydraulic system, driven by an electric motor and synchronized with the means for rotating the workpiece, for moving a tool carriage in a lathe. U.K. Pat. No. 1,217,326 describes the driving of hydraulic motors by means of a circuit in which the pressure and rate of flow are controlled by a variable-capacity pump which is in turn driven by a motor.
However, until now there has never been a really satisfactory solution to the problem of driving an end-machining tool in an automatic lathe machining workpieces at the end of a bar when the lathe is numerically controlled.
It is therefore an object of this invention to provide an improved driving system which meets this need.
To this end, in the driving system according to the present invention, the improvement comprises an electric motor simultaneously driving the spindle and a pump, the pump being incorporroated in a hydraulic circuit which in turn comprises a hydraulic motor driving the toolholder.